In recent years, the development of materials using various coating methods has been advanced. In particular, coating techniques of a thin layer of several micrometers to several hundred nanometers are required in the fields of optical film, printing and photolithography, and coating accuracy required is raised with thinning of films, enlarging of the size of substrates and the expedition of coating speed. Particularly in the manufacture of optical films, the control of film thickness is a very important point that influences optical performances, and demand for techniques capable of realizing high coating speed while maintaining high accuracy is becoming more and more high.
Anti-reflection films are generally arranged on the outermost surfaces of display units such as cathode ray tube (CRT), plasma display panel (PDP), electroluminescence display (ELD) and liquid crystal display (LCD) to reduce reflectance by using the principle of optical interference for the purpose of preventing contrast reduction and mirroring of images by the reflection of outer light.
With the prevalence of display units having a shorter depth and a greater display area particularly as compared with conventional CRT, display units that are further precise and high in quality are come to be required. With that tendency, the planar uniformity of anti-reflection films is strongly desired. Planar uniformity used here means that the dispersion of optical performances represented by reflection preventing performance, and the dispersion of physical performances of films such as scratch resistance do not exist throughout the display area.
As the manufacturing method of an anti-reflection film, an inorganic vapor deposition such as the manufacturing method of a glare-proof anti-reflection film excellent in a gas barrier property, a glare-proof property, and a reflection preventing property using a silicon oxide film by CVD as disclosed in JP-A-7-333404 is exemplified, but the manufacturing method of an anti-reflection film by all wet coating is advantageous from the viewpoint of mass production.
However, although all wet coating using a solvent is very advantageous from the viewpoint of productivity, it is very difficult to maintain drying of solvent just after coating constant, so that planar unevenness is liable to occur. Planar unevenness used here means wind unevenness that is thickness unevenness caused by drying wind or drying unevenness attributable to the difference in drying velocity of solvent. It is an essential technique in all wet coating to increase a coating speed to improve productivity as much as possible. However, mere increase of a coating speed results in the increase of a wind speed of drying air relatively, and the planar condition is also influenced by the wind accompanied by high speed moving of the support, so that wind unevenness deteriorates. Thus, coating speed could not be increased too much so far to obtain anti-reflection films suppressed in the dispersions of optical performances and physical properties of films.
It is known that the improvement of a leveling property is effective to reduce unevenness in coating. As one means to improve a leveling property, the addition of a surfactant to a coating composition is proposed. This method is based on the mechanism that surface tension lowers and wetting of a coated matter is improved by the addition of a surfactant to a coating composition, and the variation of surface tension in film-forming process is lessened to thereby prevent heat convection, thus, the uniformity of a film is improved (Coating yo Tenkazai no Saishin Gijutsu (The Latest Techniques of Additives for Coating), compiled by Haruo Kiryu, CMC Publishing Co. (2001)). The optimal kind of surfactant varies by the compatibility with the solvent, resin and various additives in the objective coating composition, but when a solvent is used in coating, it is effective to use fluorine surfactants soluble in solvents and having the highest ability of lowering surface tension. In general, fluorine surfactants are compounds having in the same molecule a perfluoroalkyl (Rf) group for realizing the function of reducing surface tension, and a medium-philic group contributable to the affinity with various components for coating and film forming materials when the surfactants are used as additives. Such a compound can be obtained by the copolymerization of a monomer having a perfluoroalkyl group and a monomer having a medium-philic group.
As the representative examples of monomers having a medium-philic group copolymerized with a monomer having a perfluoroalkyl group, poly(oxyalkylene) acrylate and poly(oxyalkylene) methacrylate are exemplified.
However, although drying unevenness and wind unevenness are bettered by the use of conventional fluorine surfactants, periodic thickness unevenness (hereinafter referred to as “level unevenness”) occurs perpendicularly to the traveling direction of a transparent support, which is a problem that degrades the grade of coated anti-reflection films.